Changing a jpg file to tsk?

[2sa]

I have been messing around with some jpg files as today backgrounds from my pictures stored on my WX....How do I change those into a tsk file so they will appear as theme options on the Appearance tab? Is is as simple as changing the extension and dropping it into the Windows directory? Also, will the .tsk file take less storage?

Attached is a example of what I am working on.....Thanks in advance